In a usual locking clamp, the axis of rotation of the locking clamp runs beneath the foot of the stock rail so that in the closed position a force component that is approximately parallel to the slide chair acts on the switch blade and this ensures that it closes. Since the point of rotation of the clamp is beneath the foot of the stock rail, this also causes the blade to twist, and this causes the stock rail and the switch blade to move apart. The amount of such twisting becomes greater, the greater the force that is introduced. In addition, when a force is applied in this way, it is not possible to hold the switch blade down without using additional means.
U.S. Pat. No. 4,921,189 (EP-A 0 320 636) describes a switch locking system in which an L-shaped locking clamp runs above the rail foot of the switch blade. The resultant force that causes the switch blade to rest against the stock rail, and which passes between the shaft and the locking clamp support that runs beneath the foot of the stock rail, is such that the vertical force component intersects the base on which the switch blade is moved back and forth at such a large distance from the rail foot that the greater the resultant force, the more powerfully the locking clamp pulls the switch blade to the stock rail, and the more the switch blade will be tilted, so that a gap will open up between this and the stock rail.